Process for cooling melamine

ABSTRACT

Process for cooling liquid melamine by mixing with solid melamine or with solid inert substances or with a mixture of solid melamine and solid inert substances.

[0001] The application relates to a process for cooling liquid melamineby mixing with solid melamine.

[0002] The literature has already disclosed a multiplicity of processesfor preparing melamine (Ullmann's Encyclopedia of Industrial Chemistry,5th Edition, Vol. A-16, pp 174-179). All industrially importantprocesses begin from urea, which is reacted to form melamine, ammoniaand CO₂ either at high pressure and non-catalytically or at low pressurewith the use of a catalyst.

[0003] In the low-pressure processes, gaseous melamine is produced, inthe high-pressure processes, essentially liquid melamine is produced.Gaseous melamine present is passed, together with the off-gases CO₂ andNH₃, through a urea melt, with the off-gases cooling, the melaminedissolving in the urea and the urea being heated and fed to the reactorfor the melamine synthesis. Gaseous melamine is also produced by thehigh-pressure process of WO 95/01345 (Kemira), the melamine melt finallyobtained evaporating.

[0004] A big problem in the cooling and solidification of liquidmelamine is that a temperature difference of over 300° C. must be passedthrough, and byproducts can form in the course of this. A familiarmethod for cooling is quenching with water or with steam,recrystallization generally being necessary in order to remove thevarious byproducts. If gas, for instance gaseous ammonia, is used forquenching, very high volumes of gas must be used and circulated. Ifliquid ammonia is used for quenching, for instance as in U.S. Pat. No.4,565,867, although the heat of evaporation of the ammonia is used forcooling, likewise large amounts of gas must be circulated andcontinuously recompressed. Unexpectedly, a simple process has now beenfound in which the formation of byproducts is suppressed and in whichlarge amounts of gas do not need to be circulated and recompressed.

[0005] The invention therefore relates to a process/ for cooling liquidmelamine by mixing with solid melamine or with solid inert materials orwith a mixture of solid melamine and solid inert materials.

[0006] Suitable solid inert materials can preferably be metal particlesor glass particles, for example balls or rods of steel, in particularstainless steel, steel alloys or titanium alloys. It is also possible tocool additionally by feeding cold liquid NH₃ or gaseous NH₃ or byadditional cooling elements and heat exchangers.

[0007] To mix the liquid melamine with solid melamine, not only cansolid melamine be introduced into the liquid melamine, but also liquidmelamine can be introduced into solid melamine, or the reaction partnersencounter one another in a pressure-reduction and quenching vessel(quencher). It is preferable here if the liquid melamine is reduced inpressure on mixing. It is found to be advantageous to feed additionalNH₃ during the mixing. The melamine is preferably cooled to below themelting point of melamine.

[0008] The liquid melamine to be cooled is under a certain ammoniapressure of about from 1 to 1000 bar. Since liquid melamine, dependingon pressure and temperature, comprises byproducts such as melam, melem,melone, ureidomelamine, ammeline or ammelide, or has a tendency toeliminate NH₃, it is preferably under ammonia pressure. The higher thisammonia pressure, the lower the content of byproducts. Depending on themelamine preparation process carried out, the liquid melamine to becooled is advantageously under an ammonia pressure of from about 40 to1000 bar, preferably from about 40 to 400 bar, particularly preferablyunder a pressure of from about 60 to 300 bar.

[0009] Liquid melamine can be cooled, for example, by introducing solidmelamine into liquid melamine which is under a certain ammonia pressure.The solid melamine is heated on introduction of and mixing with themelt, while the melt cools. The ammonia pressure under which the melt iscan in this case remain the same, be increased or be decreased.Preferably, in a continuous process it remains roughly constant.

[0010] The temperature of the melt or of the resulting mixture can, ifappropriate, be decreased with the aid of additional cooling to belowthe solidification point of melamine, so that pure and solid melamine isformed in a gentle manner. If appropriate, the solid melamine formedstill remains for a certain time under ammonia pressure, and is thendepressurized.

[0011] However, it is also possible to decrease the temperature of theliquid melamine to be cooled only to the melamine solidification point,dependent on the respective ammonia pressure, or to just above it, inwhich case it is possible to add to the solid melamine ammonia also, forinstance in liquid, gaseous or supercritical state, to saturate withammonia the liquid melamine which can absorb more ammonia at lowertemperature. This procedure can also be used, for example, if the liquidNH₃-saturated melamine melt is then to be depressurized and solidifiedfor instance in accordance with WO 97/20826.

[0012] The preferred possibility for cooling liquid melamine with solidmelamine is to cool it below the solidification point.

[0013] It is possible in this case to mix the mixing partners, retainingthe existing pressure, with subsequent pressure increase or underpressure decrease. Preferably, mixing is performed with pressuredecrease.

[0014] It is possible to introduce solid melamine into liquid melamineor liquid melamine into solid melamine, or to introduce both mixingpartners simultaneously into a quencher.

[0015] According to a preferred embodiment, solid melamine is chargedinto a vessel and liquid melamine is introduced, preferably withpressure decrease. Particularly preferably, the mixing is carried out ina fluidized bed.

[0016] At the beginning of the reaction, solid melamine or foreignmaterial in the form of solid inert substances or a mixture of solidmelamine and solid inert substances is introduced into the fluidized-bedreactor and used to build up the fluidized bed. As solid inertmaterials, use is preferably made of fluidizing bodies of metals orglass, for example balls or rods of steel, in particular stainlesssteel, steel alloys or titanium alloys. The fluidized bed is maintainedby a gas, preferably ammonia. The temperature in the fluidized-bedreactor is below the melting point of melamine. Liquid melamine isinjected. The finely divided liquid melamine forms a layer over thesolid melamine particles or inert substance particles, causes these togrow and becomes solid. Owing to the agitation and friction of theparticles in the fluidized bed, melamine is continuously abraded orknocked off from the particles. The larger and thus heavier melamineparticles are discharged, for instance using a cyclone, as soon as theyhave reached a certain wanted particle size. Firstly, solid coldmelamine can, to a small proportion, be fed continuously, so that theliquid melamine can deposit and solidify on it, secondly, depending onthe mode of operating the fluidized-bed reactor and the other conditionsprevailing in the fluidized bed, solid melamine particles form even inthe gas space, which particles serve as crystallization nuclei and arecovered with liquid melamine which then likewise solidifies. In thiscase, no solid melamine or virtually no solid melamine needs to be fedfrom the outside.

[0017] The solid melamine particles and inert substance particles in thefluidized bed can be cooled, and thus the desired temperature in thefluidized bed set, in a plurality of ways, for example by built-incooling elements, by feeding solid cold melamine, by inert particleswhich, if appropriate, are ejected and, after external cooling, returnedto the fluidized bed, by feeding cold liquid NH₃ or gaseous NH₃, by thetemperature and rate of the gas stream which maintains the fluidizedbed, and by the enthalpy of evaporation of the ammonia present in theliquid melamine. Some of this ammonia is recirculated to cool andmaintain the fluidized bed. The ammonia is cooled, preferably beforebeing returned to the fluidized bed, and if appropriate is liquefied.The other portion of the ammonia released can, depending on the existingpressure in the fluidized bed, be returned to the melamine/urea processin the gaseous or liquid state. Here, a particular advantage of theprocess according to the invention is displayed, since no additional gasor ammonia not originating from the melamine/urea process is necessaryto maintain the fluidized bed.

[0018] The temperature existing and maintained in the fluidized bed,depending on the procedure chosen, can fluctuate in a large rangebetween room temperature and to just below the pressure-dependentmelting point of melamine. It is, for example, from approximately 100 toapproximately 340° C., preferably from approximately 200 toapproximately 340° C., particularly preferably from approximately 280 toapproximately 320° C.

[0019] The pressure existing in the fluidized-bed reactor can likewisefluctuate in a large range, depending on the procedure chosen. It can befrom somewhat over 1 bar to just below the pressure of the melamine meltto be cooled.

[0020] Customarily, the pressure in the fluidized-bed reactor is betweenapproximately 1.5 and approximately 100 bar, preferably betweenapproximately 1.5 bar and 50 bar, particularly preferably betweenapproximately 5 and 25 bar. Above a pressure of approximately 13 bar,the excess NH₃ gas can readily be liquefied and returned to the urea andmelamine synthesis.

[0021] The NH₃ pressure above the melamine melt to be cooled canlikewise vary in a large range. Frequently, it is at the pressure of themelamine synthesis carried out in the reactor. However, it can besubstantially higher if an “aging” process is connected downstream ofthe melamine synthesis. The pressure can accordingly be up to 1000 baror up to the limits which are economic and expedient and possible interms of materials. On introducing the melamine melt into thefluidized-bed reactor, the pressure is reduced to that prevailing there,the liquid melamine being cooled and solidified. In principle, thetemperature of the liquid melamine to be cooled can vary in a largerange. It is above the melting point of melamine, dependent on therespective ammonia pressure, in a range up to approximately 450° C.,preferably up to approximately 370° C., particularly preferably up toabout 350° C. The higher the ammonia pressure, and the lower thetemperature of the melamine melt, the more ammonia is present in themelamine, and the lower is the melting point. At an ammonia pressure of300 bar, the melting point is, for example, at about 300° C., at 1 barit is at 354° C. It is therefore also possible to have melamine liquidat 300° C. present, more precisely a mixture of liquid melamine withammonia, and to depressurize it if the pressure is high enough. It isparticularly advantageous to carry out depressurization at a temperaturewhich is not essentially above the respective melting point of themelamine, and to mix it with the solid melamine. This cooling to justabove the melting point of the melamine is preferably carried out byfeeding cold liquid ammonia or gaseous or supercritical ammonia. Theammonia present in the liquid melamine likewise contributes to coolingin the subsequent depressurization and counteracts the enthalpy ofmelting released on solidification of the melamine.

[0022] If solid melamine is fed, the temperature of the solid melaminecan be at any described value below the melting point of melamine, agreater temperature difference between solid melamine and liquidmelamine to be cooled having a greater cooling effect. Advantageously,melamine fine contents produced can be returned to the fluidized-bedreactor, and serve there as crystallization nuclei.

[0023] A further possibility for temperature control is injecting liquidammonia.

[0024] The temperature of the solid melamine to be discharged can be anyvalue below the melting point of melamine, preferably it is belowapproximately 320° C., particularly preferably below about 300° C. Thesolid melamine, which can further be subjected as desired to a heattreatment under ammonia pressure (tempering) is then furtherdepressurized and cooled to room temperature in any desired manner.During tempering, the liquid melamine is cooled to below the meltingpoint which is dependent on the respective ammonia pressure and is thenkept for approximately 1 min to 20 h under an ammonia pressure of fromabout 5 to 1000 bar at a temperature of approximately 100° C.,preferably approximately 200° C., to below the melting point dependenton the respective ammonia pressure.

[0025] The process according to the invention is preferably carried outfollowing a melamine synthesis from urea, particularly preferablyfollowing a melamine synthesis under pressure.

EXAMPLE

[0026] In a pilot plant, the melamine taken off from the reactor of aproduction plant is separated in a separator from the reaction gases(off-gases) CO₂/NH₃, stripped by 100 kg of ammonia/h in a downstreamreaction vessel at a pressure of 100 bar and then passed into an agingvessel. At an NH₃ pressure of 250 bar and a temperature of 330° C., themelamine melt was saturated with NH₃ and allowed to dwell for one hour.From the aging vessel, then, approximately 11 kg of melamine melt/h weresprayed into a melamine fluidized bed. The fluidized bed was maintainedby NH₃ gas and operated at a pressure of 25 bar at a temperature of 300°C. Solid melamine was discharged, depressurized and cooled to roomtemperature. Purity: 99.8% by weight melamine.

1. A process for cooling liquid melamine by mixing with solid melamine or with solid inert substances or with a mixture of solid melamine and solid inert substances.
 2. The process as claimed in claim 1 , wherein the liquid melamine is under an NH₃ pressure of 1-1000 bar.
 3. The process as claimed in claim 1 , wherein NH₃ is fed during mixing.
 4. The process as claimed in claim 1 , wherein the cooling is carried out to below the melting point of melamine.
 5. The process as claimed in claim 1 , wherein the liquid melamine is depressurized during mixing.
 6. The process as claimed in claim 1 , wherein the liquid melamine is saturated with NH₃ prior to the cooling.
 7. The process as claimed in claim 1 , wherein the liquid melamine, during mixing, is depressurized and cooled to below the melting point of melamine.
 8. The process as claimed in claim 1 , wherein the solid inert substances consist of metal particles or glass particles.
 9. The process as claimed in one of claims 1-8, wherein the mixing is carried out in a fluidized bed.
 10. The process as claimed in claim 9 , wherein the temperature in the fluidized bed is from approximately 100 to approximately 340° C.
 11. The process as claimed in claim 9 , wherein the pressure in the fluidized bed is from approximately 1.5 to approximately 100 bar.
 12. The process as claimed in claim 9 , wherein the fluidized bed is made up of solid melamine.
 13. The process as claimed in claim 9 , wherein the fluidized bed is made up of solid melamine and solid inert substances.
 14. The process as claimed in claim 9 , wherein the fluidized bed is maintained by a gas, preferably by ammonia.
 15. The process as claimed in one of claims 1 to 14 , wherein the liquid melamine is cooled to below the melting point and is then allowed to dwell for from approximately 1 min to 20 h under an ammonia pressure of from approximately 5 to 1000 bar at a temperature of from about 100° C. to below the melting point.
 16. The process as claimed in one of claims 1 to 15 , wherein it is carried out following a synthesis of melamine from urea carried out under pressure. 